Although high-capacity optical transmission systems are desired because of an increase in traffic in the Internet or the like, the band over which transmission through optical fibers is possible is currently used up to approximately a limit, and thus transmission schemes that are capable of transferring a greater amount of information over the same transmission band and that provide high spectral efficiency are being demanded. Even in the state in which signals of adjacent channels overlap with each other, OFDM (Orthogonal Frequency Division Multiplexing) can transfer the signals without interference by taking advantage of the orthogonality of optical frequencies, it can increase spectral efficiency, and thus it is expected as a next-generation optical communication scheme.
As a method for receiving optical OFDM signals, coherent reception is performed and subcarriers are separated using a technique employed in wireless technology (refer to, for example, non-patent document 1). In addition, as another method for receiving optical OFDM signals, a method is performed in which subcarriers are separated in the optical domain using a Mach-Zehnder delay-interferometer and direct reception (square-law detection) is performed (refer to, for example, patent document 1 and non-patent document 2).    [Non-Patent Document 1] S. L. Jansen, I. Morita, and H. Tanaka, “16×52.5-Gb/s, 50-GHz spaced, POLMUX-CO-OFDM transmission over 4,160 km of SSMF enabled by MIMO processing”, ECOC2007, PD1.3, Berlin, Germany    [Patent Document 1] Japanese Patent No. 3789784    [Non-Patent Document 2] A. Sano, H. Masuda, E. Yoshida, T. Kobayashi, E. Yamada, Y. Miyamoto, F. Inuzuka, Y. Hibino, Y. Takatori, K. Hagimoto, T. Yamada, and Y. Sakamaki, “30×100-Gb/s all-optical OFDM transmission over 1300 km SMF with 10 ROADM nodes”, ECOC2007, PD1.7, Berlin, Germany